Technology Guide

The KLa Difference: Technology, Experience, Knowledge, and Service

KLa Systems, Inc. 31 Mill Street/PO Box 940, Assonet, MA 02702 Tel: (508) 644-5555 Fax: (508) 644-5550 www.klasystems.com

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Table of Contents

Section 1: KLa Technology

Page 1 KLa Technology KLa Jet Aerator

Page 2 KLa Jet Aeration System Key Performance Factors Process Benefits

Page 3 Project Life Cycle Benefits Oxygen Transfer Performance Page 4 Aeration System Design

Page 5 (SOR) Standard Oxygen Requirement Calculation and Nomenclature

Page 7 Technology Applications Maintenance Considerations Page 9 KLa and Alpha

O2 Transfer Rate and Nomenclature Mass Transfer Coefficient (KL) Page 10 Interfacial Area per Unit Volume (a) Overall Mass Transfer Coefficient (KLa) Page 11 Design Alpha-Enhancement and Depression Page 12 Foam

List of Figures: Figure 1: KLa Jet Nozzle Figure 2: Manifold Jet Aerator Figure 3: Radial Jet Aerator Figure 4: Jet Aeration System Figure 5: Standard O2 Utilization Vs. Jet Air/Liquid Flow Ratio (10-50 ft. SWD) Figure 6: Aeration Efficiency Vs. Jet Air/Liquid Flow Ratio (10, 20 & 30 ft. SWD) Figure 7: Aeration Efficiency Vs. Jet Air/Liquid Flow Ratio (Performance Range)

Section 2: KLa Specifications

Section 3: KLa Drawings

Section 4: Other

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KLa Systems – Jet Aeration Technology

KLa Technology KLa jet aerators transfer oxygen by simultaneously introducing large volumes of high kinetic

energy liquid and air through a series of jet nozzles (Figure 1). The high velocity liquid exits

the inner, primary jet and rapidly mixes with the in-coming air in the outer jet. The intense

mixing and high degree of turbulence rapidly dissolves the air into a fine dispersion of gas and

liquid. This high velocity, oxygen rich cloud is discharged from the outer nozzle where the

plume mixes with and entrains the surrounding liquid. Each individual jet plume travels

horizontally along the basin floor prior to the vertical rise of the gas bubble column to the

surface.

In biological treatment processes, the motive liquid to the jet is furnished by recirculating the

mixed liquor using centrifugal pumps. Low-pressure air is supplied to the process via

compressors or blowers.

KLa Jet Aerator KLa jet aerators consist of two general configurations of jet nozzles mounted on specific

distribution systems. Both jet aerator designs are supplied as fabricated, monolithic units.

Manifold type jet aerators locate jet aeration nozzles (jets) on either one or both sides of a

liquid distribution pipe. The jet nozzles are attached to an air distribution pipe by individual air

ducts that serve as both an air source for the jets and support the air distribution pipe. They

are typically supplied in lengths of up to 45-ft. (13.7 m.) with the jet nozzles, air ducts, liquid

distribution manifold and air distribution manifold supplied as a single unit. For export

containers, the maximum length is just under 40-ft. (12 m.)

Radial jet aerators distribute jets uniformly around the circumference of a central, pressurized,

tank type chamber. As a rule, the jet manifold is used in larger scale applications, and the

radial aerators are limited to smaller scale biological processes utilizing circular tanks. The two

types of jet aerators are shown in Figure 2 and 3.

The aerator configuration, number of jet nozzles and nozzle spacing is designed to maximize

oxygen transfer and mixing. Effective spacing of the nozzles and aerators will insure uniform

oxygen transfer throughout the basin as well as optimizing the overall mixing process.

Fiberglass reinforced plastic (FRP) is the material of construction of the jet aerator. FRP is not

only an economical material, but also it is durable, lightweight, highly corrosion resistant, and

easily assembled in the field. Typically, the piping for the aerators or air/liquid distribution

system is machine filament wound. The jet nozzles, radial chambers and air ducts are

fabricated by custom contact molding processes. The assembly process for the connection of

the jets and air ducts to the aerator piping or radial chambers is via hand lay-up manufacturing

methods. The selection of the various grades of FRP resins, piping thickness and design

pressure ratings as well as different abrasion liners are dictated by the specific application.

The KLa Systems standard jet aerator specifications are included in this catalog. Structural

support systems are fabricated from various grades of 300 series stainless steel and

occasionally epoxy coated steel. Jet aeration and mixing systems fabricated entirely of

stainless steel are available, although more costly then FRP systems.

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KLa Jet Aeration System The KLa Jet aeration system includes the individual jet aerators and in-basin piping system,

often along with the supply of air blowers and liquid recirculation pumps. Other jet system

components include: back-flush system, out-of-basin air and liquid piping systems, isolation

valves, expansion joints, pressure gauges, air flow measurement devices and instrumentation,

supports, fittings, dissolved oxygen control system, motor control center, and acoustical

enclosures. A typical jet aeration system is shown in Figure 4.

For the supply of air, rotary positive displacement blowers, multi-stage centrifugal blowers, and

single-stage turbine type compressors and accessories are typically supplied. End suction

centrifugal pumps, vertical submersible pumps or vertical propeller pumps are usually

furnished for the supply of jet motive liquid. The pumping systems are configured as a

recirculation loop where the mixed liquor is directly withdrawn from the basin and pumped back

into the process via the liquid distribution pipe or manifold. The proper design of the liquid

recirculation system is critical to insuring that the there is a 33 ft./sec. (10 m/s) liquid velocity

continuously pumped through the inner jet nozzle.

Key Performance Factors The high velocity stream, in combination with the introduction of air, is the key feature that

makes the technology unique, and the resultant hydrodynamic conditions contribute to the

superior oxygen transfer and mixing performance as follows:

1. The highly turbulent, fine bubble cloud creates an enormous gas/liquid interface

inside the jet and along the horizontal plume, resulting in high mass transfer

coefficients (KLa).

2. The transfer of horizontal momentum from the jet to the surrounding liquid,

combined with rising bubble columns, provides superior process mixing.

Process Benefits These two factors translate into many of the process benefits of the jet aeration systems,

including:

1. High aeration efficiency

2. Higher percentage of oxygen

utilization

3. Superior mixing and process

control

4. Independent control of

mixing and oxygen transfer

Project Life Cycle Benefits These process benefits, along with the physical features of the jet aeration system,

result in significant improvements to the aeration system life cycle such as:

1. Reduced capital costs

2. Lower energy costs

3. Lower Maintenance Costs

4. Small plant footprint

5. Reduced aerosols

6. Reduced volumes of off-gas

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KLa Systems-Oxygen Transfer Performance An important aspect of jet aeration is that the device has two components of mass flow that

give it unique oxygen transfer performance characteristics. The jets use continuous mixed

liquor recirculation as the motive fluid for dissolving the gas stream and transferring momentum

to the bulk liquid. The secondary fluid (air) is introduced over a range of flow rates depending

upon the oxygen transfer and mixing requirements of the process.

The percentage of standard oxygen utilization (a.k.a. standard oxygen absorption efficiency or

standard oxygen transfer efficiency) is affected by the jet air/liquid flow ratio and liquid depth.

This is illustrated in Figure 5, with two very important characteristics of the series of curves:

The percentage of oxygen utilization increases with reactor depth. This phenomenon is

related to Henry’s law of partial pressures, but is more pronounced with jet aerators

(when compared with other diffusers) due to the ability of the high velocity liquid stream

to dissolve more of the compressed gas into solution at greater liquid depths.

As air/liquid ratio is increased, the percentage of oxygen utilization decreases.

Conversely, as ratio is decreased, the percentage of oxygen utilization increases. This

characteristic is based on the fact that one component of mass flow (liquid side) is held

constant while airflow is varied. At a constant liquid velocity, the formation and size of

the gas bubbles (which is directly related to the percent standard oxygen utilization) is

primarily controlled by the amount of airflow. Finer (smaller) bubbles are formed at low

airflows, and medium to coarse bubbles are formed as airflow increases towards the

maximum air/liquid ratio.

Varying the air/liquid ratio to a jet system is accomplished in the field by throttling the airflow.

This results in visible changes in the jet plume length. At lower airflows the motive fluid carries

the plume further along the horizontal plane, resulting in a larger mixing zone. At higher

airflows, the lower density and greater buoyancy of the gas/liquid cloud results in a shorter

horizontal component of the jet plume.

Figure 6 shows the relationship between Standard Aeration Efficiency and Jet Air/Liquid Flow

Ratio as a function of reactor depth. These curves are widely published, with their

characteristic “hump” at the optimum air/liquid ratio. KLa jet aeration systems are designed

near or at the optimum air/liquid ratio based on meeting the average oxygen requirement of

the process. Increasing or decreasing the ratio by throttling the airflow allows the flexibility to

meet minimum and maximum oxygen requirements. This feature is unique to jet aerators, is

easily automated with dissolved oxygen sensors, and improves the ability of a plant operator to

maintain process control.

The series of curves reveals the improvement in energy efficiency with increased reactor

depth. As the depth increases, the percentage of oxygen utilization increases resulting in a

decrease in the amount of jet nozzles required to meet the standard oxygen requirement. A

reduction in the number of jets decreases the liquid recirculation and airflow requirements, with

the effect being a reduction in overall energy usage (and capital cost of the aeration system).

Another characteristic of the jet aerator is a shifting to the right of the optimum air/liquid ratio

as reactor depth increases. This is due to the high velocity liquid stream more efficiently

dissolving larger volumes of compressed gas at increased depths.

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An important feature of the jet aeration system as it relates to Standard Aeration Efficiency is

shown in Figure 7. This curve shows the relationship between Standard Aeration Efficiency

and Jet Air/liquid Flow Ratio at 20-ft. (6 m.) and 40-ft. (12 m.) depths. The primary point of the

graph is to depict that, in the “real world,” these performance curves are not thin lines, but

broad ranges due the wide variation in the efficiency of the prime movers. The selection of

liquid recirculation pumps and air blowers are extremely important in optimizing the aeration

efficiency. Pump hydraulic efficiency typically ranges from 65-85%, while blower adiabatic

efficiency ranges from 60-80%.

It should be pointed out that selecting the prime mover based solely on efficiency has an

impact on the capital cost of the system. For example, substituting a low efficiency positive

displacement blower with a high efficiency turbo compressor can result in a increase in the

aeration system capital cost by 50%. The affect of proper selection of pumping systems is

more complex in terms of proportioning hydraulic efficiency to capital cost, but equally

important.

The KLa jet system design will always take into consideration project life cycle analyses, and

carefully balance the energy and capital costs in order to determine the optimum solution for

each project.

KLa Systems -Aeration System Design Aeration equipment suppliers rate the performance of their equipment by its oxygen transfer

capacity in clean water (SOTR) at 0 mg/l dissolved oxygen, one atmosphere pressure, and 200

C. In aeration system design, the goal is to convert the engineer’s actual (a.k.a. field or

process) oxygen requirement (AOR) to a Standard Oxygen Requirement (SOR) using the

following formula:

STANDARD OXYGEN REQUIREMENT (SOR)

SOR = AOR

((β x CWALT

x DC) - CL)

α θ (T

W -20)

( C*20 x DC)

SOR = Standard Oxygen Requirement, (lb or kg/hr)

AOR = Actual (Process) Oxygen Requirement, (lb or kg/hr),

Also known as OTRF per ASCE

α = Alpha factor, ratio of oxygen transfer coefficient (KLa)

of the wastewater to that of tap water

β = Beta factor, ratio of oxygen saturation of the wastewater

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to that of tap water

C WALT

= Surface saturation dissolved oxygen

concentration in clean water at the

wastewater temperature (TW) and basin elevation, mg/l

DC = Depth Correction Factor, Dc = Water Depth, ft. + 1

100

CL = Residual Dissolved Oxygen concentration,

Typically is 2 mg/l

C*20 = Surface saturation dissolved oxygen

concentration in clean water at 20°C and

one atmosphere

= 9.09 mg/l (per ASCE)

θ = Theta Factor, Temperature correction coefficient = 1.024

Tw = Wastewater Temperature (°C)

Once the SOR is determined, clean water performance data (i.e. standard conditions) is applied in

order to develop the system design. It is the role of the aeration system supplier to make sure that

the recommended design delivers an SOTR that meets or exceeds the SOR.

In review of the above equation, it can be seen that alpha factor is directly proportional to the

SOR calculated for each design. It is the only parameter that is device specific, and has a

major impact on both the size of the aeration system, and the power required to operate it.

Aeration systems consume approximately 70% of the power to run a wastewater treatment

plant, and the selection of the design alpha factor, as well as understanding its relationship to

the actual operating conditions, is one of the most important considerations with respect to the

system design.

In comparison to other diffused aeration technologies, jet aeration systems yield a higher alpha

value. This is especially true in the biological treatment of production plant effluent from

various pulp/paper, pharmaceutical, chemical and food processing plants. This characteristic

of the jet aerator is due to the hydrodynamic conditions produced by the high velocity,

turbulent jet plume in combination with some wastewater constituents common to these

industries. A detailed description of the mechanism for enhancing or depressing the alpha

factor can be found at the end of this section.

In aeration system design, a higher alpha factor results in a lower design Standard Oxygen

Requirement (SOR). The following table shows commonly accepted values of alpha for

various aeration devices treating wastewater from industry, along with the affect on the design

SOR, and power required.

Stan

dard

Oxy

gen

Util

izat

ion

vs. J

et A

ir/Li

quid

Flo

w R

atio

(10

to 5

0 ft.

SW

D)

50-ft

Sta

ndar

d O

xyge

n U

tiliz

atio

n ( %

)

96

80

64

48

32

16

0

10-ft

20-ft

30-ft

40-ft

0

.8

1.6

2

.4

3

.2

4.0

A

ir/Li

quid

Flo

w R

atio

Figu

re 5

Sta

ndar

d A

erat

ion

Effi

cien

cy

(Lbs

. O2/b

hp-h

r)

3.0

2.5

2.0

1.5

1.0

0.5

4.8

4.0

3.2

2.4

1.6

0.8

Sta

ndar

d A

erat

ion

Effi

cien

cy

(Kgs

. O2/b

kw-h

r)

10-ft

30-ft

20-ft

Aer

atio

n Ef

ficie

ncy

vs. J

et A

ir/Li

quid

Flo

w R

atio

(1

0, 2

0 &

30

ft. S

WD

)

0

.8

1.6

2.4

3

.2

4.0

A

ir/Li

quid

Flo

w R

atio

Figu

re 6

N

ote:

For

Liq

uid

dept

hs in

Exc

ess

of 3

0-ft,

C

onta

ct K

La S

yste

ms

Sta

ndar

d A

erat

ion

Effi

cien

cy

(Lbs

. O2/b

hp-h

r)

Sta

ndar

d A

erat

ion

Effi

cien

cy

(Kgs

.O2/b

kw-h

r)

Aer

atio

n Ef

ficie

ncy

vs. J

et A

ir/Li

quid

Flo

w R

atio

(P

erfo

rman

ce R

ange

)

3.5

3.0

2.5

2.0

1.5

1.0

0.5

5.6

4.8

4.0

3.2

2.4

1.6

0.8

40-ft

SW

D

20-ft

SW

D

0

.8

1.6

2

.4

3

.2

4.0

A

ir/Li

quid

Flo

w R

atio

Figu

re 7

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Type of Aerator Alpha Relative SOR (%) Relative Power (%)

Jet 0.90 100 100

Coarse Bubble 0.70 129 157

Fine Pore 0.40 225 142

Surface 0.90 100 130

Note that the SOR is directly proportional to alpha, but the power required is not.

This is due to the differences in the standard oxygen utilization (a.k.a. SOTE) between the

devices. For example, in 20-ft. (6 m.) deep aeration basins, a fine pore diffuser will have a

standard oxygen utilization efficiency of 31% versus 26% and 16% for jets and coarse bubble

diffusers respectively. These differences, combined with the affect of the alpha value

contribute to the overall energy requirement of the aeration system.

In the design of aeration systems for sewage treatment plants, the differences in alpha

between jets and fine pore diffusers are not as great. In many instances the total power usage

for fine pore technology will be less then jet aerators, and this has contributed greatly to the

success that fine pore diffusers have had in the municipal market.

For municipal applications, particularly where nutrient removal is a design consideration, the jet

aerator’s ability to mix the basin contents with the airflow turned off should be emphasized.

This feature allows for superior process control, and is a key advantage in applying the

technology to SBR, Oxidation Ditch and other nutrient removal processes. Another important

feature of jet aerators in municipal applications is the frequency of cleaning the in-basin

components and the design life of the system. Jet systems are self-cleaning (see Back-Flush

system, pg. 7) and have a proven 20-year design life, which is not the case with submerged

diffuser technology.

KLa Systems – Technology Applications The following list shows the main applications of the KLa Systems jet aeration technology. This

list can be generally broken down into three categories (activated sludge processes, aerobic

biosolids treatment processes and equalization/holding). For industrial bioprocesses, the

complete mix activated sludge process, equalization and sludge holding are the most popular

applications typically with reactor depths in the 20-30-ft. (6-9 m.) range. For municipal

applications, oxidation ditches, SBR and other nutrient removal processes are the most

popular applications. Reactor depths here are typically in the 12-20-ft. (3.5-6.0 m.) range. The

ASB process is generally found in the pulp/paper industry, where land is plentiful and

inexpensive. In northern climates, jet aeration systems are applied to these massive basins

due to their ability to provide superior mixing without severe loss of reactor temperature or the

need for full floor coverage. The aerobic treatment of biosolids is regaining popularity, with

conventional (mesophillic) aerobic digestion taking a backseat to auto thermal thermophillic

aerobic digestion (ATAD). This is an excellent application of the technology, with special

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emphasis on both superior oxygen transfer and mixing under very high (> 3.0 %) solids

concentrations and reactor temperature (> 55 degrees C).

Jet Aeration Applications

Complete Mix Activated

Sludge

Plug Flow Activated Sludge

Oxidation Ditches (CLR)

Sequencing Batch Reactors

Pure Oxygen

ASB

ATAD

Aerobic digestion

Equalization

Sludge Holding

KLa Jet Aeration System-Maintenance Considerations There are three main components to the jet aeration system: in-basin aerators with piping

system and supports, liquid recirculation pumps with accessories, and air blowers with

accessories.

Blowers: The routine maintenance required for the air blowers are minor, and will generally be

per the manufacturers O&M manual. However, due to the large openings of the jet aerator,

the inlet filters will require less maintenance or replacement as compared to blowers furnished

for fine pore diffused aeration systems.

Pumps: When applying pumps for jet service, the primary maintenance concern is with

leakage of the pumped liquid. For dry pit applications, the KLa pumping system is designed to

include a mechanical seal. Single seals with hard faces and clean water flushing are

recommended over double seal arrangements due to the need for positive flushing of biosolids

off the seal faces, which also serves to cool the seal under continuous pumping service. In

most of our industrial activated sludge applications, the jet pumps are furnished with dynamic

seals with excellent results. Dynamic seals consist of an expeller that continuously repels

pumped fluid from around the seal area while the pump is running, and a flexible, static seal

which prevents leakage during times when the pump is not running. Single seals with clean

water flush systems typically promote a seal life of 2 to 4 years. Dynamic seals have been in

jet service for over 12 years without replacement.

When submersible pumps are furnished for liquid recirculation, seals and an operator friendly

retrieval system are the primary maintenance considerations. These pumps are equipped with

tandem mechanical seals, which typically run, trouble-free, for 12 –24 months under

continuous duty service with recommended replacement every 18 months.

Back-Flush System: One of the biggest drawbacks of most submerged aeration systems is

the requirement to drain the basin for routine cleaning of the diffusers.

This is not the case with jet aerators. The KLa jet system is available with an optional Back-

Flush System that allows for cleaning the jets without the need for basin drainage or to have

an operator enter the basin. Back-flushing the jets are conducted essentially with an airlift

pump that reverses the flow through the inner jet nozzle. The driving force for the reverse flow

through jets is the pressure differential created by the lower density of the Back-flush stream

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compared to the surrounding liquid. This powerful flushing action removes any debris that may

plug the inner jet nozzle.

The Back-Flush operation consists of:

1. Stop the airflow to the aerator.

2. Turn off the recirculation pump to the aerator.

3. Slowly open the Back-Flush valve.

4. Slowly start the airflow back to the aerator, until there is a smooth flow of air and liquid

exiting the Back-flush valve.

5. Continue flushing for 5 minutes.

6. Stop the airflow again.

7. Restart the jet pumps.

8. Restart airflow to the aerator, regulate to the original setting.

The Back-Flush is typically included in aeration system designs for municipalities and pulp mills

where heavy solids can occasionally enter the aeration basin. In many of the treatment plants

built over the past 5 years, the application of fine screens (less then 6 mm openings) for pre-

treatment has resulted in jet aeration systems that never experience plugged jet nozzles.

A typical Back-Flush operation takes only a few minutes, and is easily automated. The KLa

Back-Flush system consists of a riser pipe connected to the liquid header or distribution pipe,

supports, and a valve with operator. For deep aeration tank applications, the Back-Flush

System requires special design considerations to protect the piping system from excess

stresses.

KLa and Alpha In order to understand alpha, it is important to review the basis for the oxygen transfer rate

equation (which is the basis for the SOR equation). Among the theories describing oxygen

transfer, the most widely accepted theory is the existence of two films (a thin film on each side

of the gas/liquid interface), and the resistance to transfer of solutes across the gas and liquid

films.

The oxygen transfer rate can be expressed as:

N = KLa (CS – CL)

N = Mass of oxygen transferred per unit time

KLa = Overall mass transfer coefficient, comprising

KL and a

KL = Liquid film mass transfer coefficient

a = Interfacial area per unit volume (A/V)

CS = Saturation concentration of oxygen at the gas/liquid interface

CL = Concentration of oxygen in the bulk liquid

The above equation is commonly viewed as:

Oxygen Transfer Rate = KLa x Driving Force

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In aeration system design and testing, the combined coefficient, KLa, is used due to the

logistical challenges in measuring ‘KL’ and ‘a’ separately, as well as the fact that KLa can be

determined via non-steady state oxygenation tests using a non-linear regression data

reduction technique.

Mass Transfer Coefficient (KL)

Many of the early studies of oxygen transfer theorized that liquid film resistance to molecular

diffusion of oxygen into bulk solution was the controlling parameter in oxygen transfer.

The principal limitation to the theory is the assumption of a “steady-state” transfer across the

gas/liquid interface. Dankwertz introduced the surface renewal theory that defines the liquid

film coefficient by the following equation:

KL = (DL r)1/2

KL = Liquid film mass transfer coefficient

DL = Molecular diffusion coefficient

r = Surface renewal rate (average frequency that the liquid film

is replaced by liquid from the bulk solution)

The key hypothesis is the assumption of “non-steady state” mass transfer. For turbulent

conditions, eddies from the bulk liquid move to the gas/liquid interface and undergo short non-

steady molecular diffusion, and are then displaced from the interface by subsequent bulk

liquid. While under laminar conditions, no bulk liquid displacement takes place.

The molecular diffusion coefficient ‘DL’ is greatly affected by the presence of chemical

compounds (surfactants and other organic and inorganic chemicals) and the surface tension at

the gas/liquid interface. The surface renewal rate ‘r’ is affected by the hydrodynamic

conditions created by the type of aeration device used and the energy input (mixing intensity)

to the process. Under laminar (low turbulence) conditions ‘KL’ is controlled by molecular

diffusion ‘DL’. Under turbulent conditions, ‘KL’ is controlled by surface renewal ‘r’.

Interfacial Area per Unit Volume (a):

The effects of bubble size, interfacial area per unit volume and oxygen transfer have been

studied for the past forty years. Aeration devices are typically rated as either coarse, medium

and fine bubble systems. As bubble size decreases, the interfacial area per unit volume ‘a’

increases resulting in greater oxygen transfer rates.

In biological processes, surface-active agents and dissolved salts greatly affect gas bubble

size and the corresponding surface area available for mass transfer. Surface active agents

lower the surface tension of the liquid resulting in a significant reduction in bubble diameter

thus increasing the value of ‘a’ for the overall mass transfer coefficient KLa. Dissolved salts

inhibit the coalescing affect of the bubble plumes or clouds allowing the fine bubble formation

to maintain its small size, resulting in optimum mass transfer conditions throughout the

10

entrainment process and vertical bubble rise. Some other organic compounds such as

phenols, alcohols and carboxylic acids have the same affect as dissolved salts.

Overall Mass Transfer Coefficient (KLa):

It is important to consider that several of the above factors that affect the liquid film mass

transfer coefficient ‘KL’ also impact the surface area to volume ratio ‘a’. In particular,

wastewater characteristics (dissolved salts, surfactants and other organic compounds), the

type of aeration device, and the degree of turbulence and mixing intensity will play a major role

in the oxygen transfer process.

It has been shown that the presence of certain organic chemicals (surfactants for instance) will

have the following affect on the overall mass transfer coefficient, KLa.

1. Increased resistance at the gas/liquid interface, which impedes molecular diffusion, DL,

and thus reduces the liquid film coefficient ‘KL’ and overall mass transfer coefficient,

KLa.

2. Increased interfacial area per unit volume ‘a’ which increases the overall mass transfer

coefficient, KLa.

The hydrodynamic conditions created by the aeration device and the mixing intensity to the

process determine whether or not the KLa value is depressed or enhanced in the presence of

surfactants as follows:

1. Under laminar (low turbulence) conditions, molecular diffusion controls the liquid

film coefficient ‘KL’. In the presence of surfactants the increase in interfacial

area, a, is negated by the resistance to molecular diffusion (due to the

accumulation of organic molecules at the surface of the film). This leads to the

subsequent reduction in the values of ‘DL’ and’KL’, and a depressed value of

KLa.

2. Under turbulent conditions the liquid film is continually disrupted (or renewed) so

that the surfactant molecules do not become diffusion barriers. The surface

renewal rate ‘r’, controls the value of the liquid film coefficient ‘KL’, and the

increase in interfacial area ‘a’ is not negated by the resistance to molecular

diffusion, resulting in an enhanced value of KLa.

In the presence of high dissolved salt concentrations (0.5-3.0 %), there are often enhanced

values of KLa and alpha due to the maintenance of very small gas bubbles from initial

formation through the buoyant rise.

Design Alpha-Enhancement and Depression Since Alpha is defined as:

11

KLa (wastewater)

KLa (clean water)

The determination as to whether the value of alpha is enhanced or depressed by the

wastewater constituents or the type of aeration device (and the hydrodynamic conditions it

produces) is a function of comparing the KLa values in wastewater to those in clean water. In

certain industries, it is not uncommon for the alpha factor to be in excess of unity, and there

are many jet aeration installations around the world producing these results continuously. This

phenomenon is a result of the wastewater characteristics common to many production facilities

containing a combination of high dissolved salts and/or surfactants. The result is ideal

conditions for high “dirty water” KLa values (i.e. extremely fine bubbles and no diffusion

barrier). Fine pore diffusers typically will have depressed alpha factors under the same

conditions. This is because the production of very fine bubbles under laminar transfer

conditions can not over come the diffusion barrier. However, in wastewater with low organic

content and the high TDS levels, fine pore diffusers have produced enhanced alpha values.

Surface aerators have produced elevated alpha values under similar wastewater conditions

where jet aerators tend to perform above expectation.

Jet systems applied to long sludge age processes with high bacterial cell concentrations

(membrane bioreactors and ATAD for example) often produce high alpha factors. This has

been attributed to the release of extra-cellular enzymes and their surfactant-like

characteristics.

Without extensive pilot testing, KLa Systems does not recommend selecting a design alpha

factor above 0.90. In these cases, KLa will incorporate a flexible, air blower and pumping

system design to insure that the system can be efficiently “turned down” should the apparent

alpha factor greatly exceed the design value.

For large-scale aeration projects, pilot testing (under actual or simulated bioreactor

concentrations) is recommended to determine an appropriate design alpha factor. Alpha test

units in the 500-1000 gallon range have been shown to furnish good scale-up data for jet

aeration system design.

Foam An interesting challenge that has developed from jet aeration applications where high alpha

factors have been documented has been the characteristic of foam formation during the

aeration process. This is typically attributed to: the low surface tension of the mixed liquor, the

very stable fine bubbles that exist under high TDS levels, the degree of turbulence generated

by the jet, and the mixing intensity of the process. Under these conditions, nuisance foaming

events are not uncommon, nor are the use of anti-foams or de-foamers by plant operators to

combat these occurrences.

Note that there are other types of foams that form due to the nature or mode of operation of

the biological process (nutrient removal processes and ATAD for example). These “biological”

foams are often reduced or minimized by modifications to the process operating strategy rather

then the use of chemicals.

12

The chemical additives used to combat foam are also surfactants and are classified as anionic,

cationic and non-ionic. De-foamers and anti-foams are used differently. De-foamers are

typically added during and after a foaming event, destroying the existing foam and preventing

further foam production for a period of time. Anti-foams inhibit foam production by preventing

the formation of fine bubbles and accelerating the coalescence of the bubbles as they exit the

jet aerator and enter the entrainment process and vertical bubble column. A secondary affect

of these coalescence-enhancing chemicals, even at concentrations as low as 3 mg/l, is the

depression of oxygen transfer (KLa ). Full-scale plants have experienced up to a 50 %

reduction in the system oxygen transfer capacity. Non-ionic de-foamers and anti-foams

appear to be the biggest oxygen transfer depressors.

Anti-foams are rarely ever recommended to combat nuisance foaming events. If chemical de-

foamer is required, there are products available that perform well without depressing oxygen

transfer. KLa Systems will work with our customers to find suitable products that mitigate

foaming, while not adversely affecting the oxygen transfer capacity of the jet aeration system